1. Field of the Invention
The present invention relates to the pressure compacting or molding of mixtures comprising plaster and a water donor, such as "wet" synthetic gypsum or a mixture of same with water itself, with "wet", as used herein, being later defined. More particularly, the invention relates to the pressure compacting of such a plaster mix wherein the plaster is synthetic and has a Blaine surface in the range of between 2,500 and 7,000 cm.sup.2 /g, the percentage by weight of the synthetic plaster in the admixture is within the range of between 30% and 99%, and the excess water needed with respect to the stoichiometric amount necessary for the ultimate setting of the composition is no greater than about 15%, i.e., between 0 and 15%, of the total weight of the mixture.
The process of the invention is, moreover, well adapted for the manufacture of modular constructional units, unitarily cast and admirably suited as building materials, in particular as bricks, squares, blocks, plates, composites, facades, casings, flooring, load-bearing beams, and the like.
The invention features an improvement in the pressure compacting of "wet" gypsum/plaster admixtures, especially the low pressure integral molding thereof into modular constructional units and the building materials resulting therefrom.
2. Description of the Prior Art
The pressure casting of mixtures of plaster and gypsum has already been proposed to this art. Netherlands Pat. No. 6,809,939 specifically describes a process which consists of mixing gypsum [hemihydrated calcium sulfate], a residuum of a certain technique for the manufacture of phosphoric acid, with "natural" plaster [semihydrated calcium sulfate] in a weight proportion of a maximum of 5/1, and of compressing this mixture in a mold under a pressure of at least 100 kg/cm.sup.2. The amount of plaster used in the mixture depends on the resultant material sought to be produced and on the water content of the residual gypsum. It is assumed, moreover, that the water comprising gypsum, said water being present in the form of water physically combined [water of hydration or crystallization] or superficially absorbed, reacts under the effects of strong pressure with the plaster in the mixture to form dihydrated calcium sulfate, the setting of which yields a material especially suited for use in the agricultural industry, for use in making bricks or plates, for acoustical applications, and for use in the cement industry.
However, the necessity for compacting the gypsum/plaster mixtures, in accordance with the aforenoted Netherlands patent application, at pressures higher than 100 kg/cm.sup.2 in order to obtain suitable materials, nonetheless markedly reduces the advantages of such a process both in terms of its practical as well as economic aspects.
Moreover, production of cast materials from gypsum/plaster mixtures exhibiting excellent mechanical properties as well as excellent surface quality is a desideratum for manufacturing economically feasible elements. By providing cast articles with a homogeneous, smooth surface, the cost of labor in subsequent finishing work is greatly reduced.
Another patent which discloses the casting of natural plaster and gypsum is French Pat. No. 2,196,299, which describes the manufacture of cast pieces from water setting mixtures comprising a special "natural" gypsum and a chemical synthetic gypsum.
Accordingly, it is an object of the instant invention to provide a process for producing plaster and gypsum based elements having very high mechanical strength.
Moreover, it is an object of the instant invention to provide a moldable composition which can provide a molded article with smooth and improved surface quality and very high mechanical strength.
It is still another object of the instant invention to provide a homogeneous and economically feasible load bearing block.
Other objects, features and advantages will become apparent to those skilled in the art upon a study of this disclosure and the appended claims.